The special flag $GUI_BKCOLOR_TRANSPARENT can be used with Label, Group, Radio, Checkbox controls to give them a transparent background.

The special flag $GUI_BKCOLOR_LV_ALTERNATE can be used with Listview control to give alternate background of the ListviewItems lines.
The odd lines will get the color set by GUICtrlSetBkColor of the Listview control.
The even lines will get the color set by GUICtrlSetBkColor of the ListviewItem control.